The present invention relates to the field of casting, and more particularly to a model for lost-pattern casting, and also to methods of fabricating shell molds and of casting using such a model.
In the description below, the terms “top”, “bottom”, “horizontal”, and “vertical” are defined relative to the normal orientation of such a mold while metal is being cast into it.
So-called “lost-wax” or “lost-pattern” casting methods have been known since antiquity. They are particularly suitable for producing metal parts of complex shape. Thus, lost-pattern casting is used in particular for producing turbomachine blades.
In lost-pattern casting, the first step is normally to make a model out of a material having a melting temperature that is comparatively low, e.g. a wax or a resin. The model is itself coated in a refractory material in order to from a mold, in particular a mold of the shell mold type. After the model of the material has been emptied out or eliminated from the inside of the mold, whence the name for such “lost-pattern” methods, a molten metal is poured into the mold in order to fill the cavity left by the model in the mold after the model has been emptied out or eliminated. Once the metal cools and solidifies, the mold can be opened or destroyed in order to recover a metal part having the shape of the model. In the present context, the term “metal” covers both pure metals and above all metal alloys.
In order to be able to produce a plurality of parts simultaneously, it is possible to unite a plurality of models in a single cluster in which they are connected together by a shaft that forms casting channels in the mold for the molten metal.
Among the various types of mold that can be used in lost-pattern casting, so-called “shell” molds are known that are formed by dipping the model or the cluster of models in a slip, followed by dusting the slip-covered model or cluster with refractory sand in order to form a shell around the model or the cluster, and then baking the shell in order to sinter it so as to consolidate the shell as a whole. It is possible to envisage dipping and dusting several times in succession in order to obtain a shell of sufficient thickness before baking. The term “refractory sand” is used in the present context to designate any granular material of grain size that is sufficiently fine to meet the desired production tolerances, and that is capable in the solid state of withstanding the temperatures of the molten metal, while also being capable of being consolidated to form a single piece while the shell is being baked.
In order to obtain particularly advantageous thermomechanical properties in a part produced by casting, it may be desirable to ensure that the metal is subjected to directional solidification in the mold. In the present context, the term “directional solidification” is used to mean that as the molten metal passes from the liquid state solid crystals are seeded and grown therein under control. The purpose of such directional solidification is to avoid the negative effects of grain boundaries in the part. Thus, directional solidification may take place in columns or it may be monocrystalline. Directional solidification in each column consists in orienting all of the grain boundaries in the same direction so that they cannot contribute to propagating cracks. Monocrystalline directional solidification consists in ensuring that the part solidifies as a single crystal, so as to eliminate all grain boundaries.
In order to obtain such monocrystalline directional solidification, the mold typically presents in the mold cavity a “starter” cavity that is connected to the mold cavity via a selector channel, as disclosed for example in French patent FR 2 734 189. While the metal is solidifying in the mold, the mold is caused to cool progressively, starting from the starter cavity, so as to cause crystals to be seeded therein. The role of the selector channel is firstly to give precedence to a single grain, and secondly to enable that single grain to advance towards the mold cavity from the crystallization front of that grain as seeded in the starter cavity.
An awkward shape in the mold cavity can constitute an obstacle to such directional solidification. Thus, in a mold cavity presenting a large horizontal projection, in particular a projection corresponding to the platform of a turbomachine blade, the solidification front can suddenly cease to advance in a substantially vertical direction and can begin advancing in the direction of the projection. Such a sudden change of direction can give rise to defects, and in particular to unwanted grains, in the proximity of the projection.
In order to avoid that, the person skilled in the art makes use of grain ducts, serving to provide the solidification front with alternative paths to horizontal projections in mold cavities, and to do so without any sudden change of direction. Nevertheless, a drawback of such a grain duct is that it makes the mold more difficult to knock out, and above all the metal branches that result from the solidification of the metal material in the grain duct subsequently need to be removed from the raw casting, thereby adding finishing-off steps that are complicated and expensive.